treat pension as a bond

A pension is no different from a part-time job. It's a side income that requires no further work (or deferred income from past work), not an investment. If it were an investment, you could sell it and buy something else or borrow against it.

So as others have said, the proper way to account for side income, whether its a pension, a job, an annuity or rental income, is just to subtract it from your gross expenses, leaving you with a net expense number to be covered by your actual investments. Any decent calculator will allow you to have cash flows that turn on or off in any particular year and allow for an inflation adjustment if its applicable. Use the free one at Portfolio Visualizer ("Financial Goals" tool) if you don't have one or can't spreadsheet it.

Trying to model it as something its not or compare it with a bond just creates confusion, complexity and inaccuracies.

As Roberto and Joe both mention, a straightforward way to think of any guaranteed income (social security, pension) is to subtract that income from your expected expenses. Even if your portfolio needs to provide only half your retirement income because a pension covers the rest, you still may want to protect against recession or lower overall volatility or generate income, so you may still want to hold bonds of different types.

Bonds serve different roles in a portfolio depending on their time to maturity. Long-term bonds provide diversification to smooth the ride during a recession when equities are down for an extended period. Intermediate-term bonds provide a similar stability to long-term bonds, but to a lesser degree. Short-term bonds provide income for living expenses.

Retail investing has come a long way from our grandparents' time. It's now possible for DIY investors to inexpensively diversify into asset classes that used to be available only to hedge fund managers or were too expensive. Diversification beyond large-caps/bonds can better protect a nest egg while also enabling higher safe withdrawal rates. The podcast that Joe mentions is all about this topic. Check out episode 190 for an introduction to modern portfolio construction, episodes 480 and 490 for discussions of the four quadrant model of diversification.

You might also plug portfolios you're considering into this tool from Portfolio Charts to compare it against more diversified portfolios. And for an introduction to how diversification works, check out this article from Portfolio Charts.

I've struggled through "how to fit a pension into my plan" , and I've come to a few conclusions.

First, I highly recommend "The Golden Albatross" by Grumpus Maximus. He was a guest on Choose FI several times a few years back, and his book was published by Choose FI. I still consider the go-to resource both for thinking through if it is worth staying for the sake of the pension and once you have the pension how to work it into your FI plan.

Second, in the pre-FI context, I think that it is really helpful to treat a future pension benefit as a bond-equivalent in terms of asset allocation planning. Earlier this year, Mr. Money Mustache published an article, The Shockingly Simple Math Behind Social Security, that walks through the concept of calculating a present value for a planned future income stream. There is also a Michael Kitces article from a number of years ago about how to value Social Security benefits as an asset. So, to me, if it works out that you have $1M of assets invested in securities and (based on your assumptions of future income, planned filing age, expected benefit, and interest rate forecast) the present value of your future Social Security benefit turns out to be $250k, then that works out to an 80% stock/20% bond split right now. Naturally, you can and should adapt the formula for your particular pensions scenario.

Finally, once you are "in early retirement" (i.e., drawing from your assets), I think it makes sense to not worry about "how much bond-equivalent value does my pension contribute to my portfolio". Rather, I think it makes sense to look at everything from a cash-flow perspective. Your pension will cover X% of your annual spend. If the remaining Y% of annual spend that you require translates to a 1% withdrawal rate, it essentially doesn't matter what your asset allocation is. If it corresponds to a 3% withdrawal rate, then you are getting close to having to consider something a bit more nuanced than 100% equities. And if it corresponds to a 5% withdrawal rate, you will likely need a more advanced sort of allocation. And the reason that this seems more appropriate to me is that while your pension income stream can be converted to a bond-equivalent value, you don't actually hold it as an asset, and you can't really "sell" it. So, the percentage of your portfolio occupied by that bond-equivalent value isn't really relevant or useful to factor in.

Anyhow, I'm not a professional, can barely be called an amateur, and am just a random person on the Internet. Take all of this with a grain of salt.

At 3.5% it doesn’t matter too much what you hold. Check out risk parity radio and learn how you can increase that to 5% or more and spend more of what you’ve worked for or maybe even leave work earlier if you don’t love it.

As to your direct question, take your expenses and deduct your pension. Then that’s what your portfolio is covering and treat your portfolio in that regards. Your pension is guaranteed future cash for future expenses, that is all.