I think you are thinking too much about this.
You can't predict the rate of return.
You can't predict the rate of inflation.
You can't predict the tax rates in 20 years.
It's not a yearly gain.
You should not have a fund by the way, but a portfolio of individual shares.
You might have some without gains and might choose to sell them first and not pay any CGT.
When you die, the CGT liability, disappears - under current rules. (These are stupid rules and I hope it's changed)
Hummm....well I am only a novice when it comes to personal finance, but let me explain my understanding...
I can't predict the future rate of return, but we do have data on historic returns. For example, we know the S&P500 and Nasdaq100 ROR average return y-o-y. I can buy a portfolio (fund was used as a synonym) that aligns with the weightings of such an index. I can then model using lower and upper bounds (bear periods and bull periods). I can do the same if I go with 60/40 equity/bonds portfolios etc. I fully accept that "past performance is no guarantee of future performance" but to not model/plan as a result seems a bit like throwing the baby out with bath water. Is it not sane to say if I invest €500k in an index that has previously yielded 7% over any 15 year period then I can assume that I will yield say 3.5% over the next 15 years. This will give me a portfolio value that is useful to know rather than not knowing....
Ditto for rate of inflation - we have all the historic data.
As for tax rate, it is an assumption that it will be the same. Out of my control of course, but again - just because I am uncertain doesn't mean I don't make a plan/model surely? A plan with 33% CGT included is better than no plan...
What is the point in saving/investing etc. if one doesn't have some idea on what the outcome of the numbers will be...? And how would you know if a small tweak to the numbers could be in your favour eg maybe that new car isn't a good idea if it could shift retirement out by 5 years...a simple model will tell you the sensitivity of such big decisions.
Finally, what do financial planners actually do if not make a year-on-year financial model/plan?
Thanks for the info on CGT with married couples - good to know!