The biggest danger I see in online calculators is that they use a flat % for the entire retirement scenario and inflation will not be flat across the board. SS, Pensions, etc will have different inflation rates. Even your investment expectations will / should change as you age and take a less active role.
In our case, both being retired and pre-pension/SS, all our current income comes from our investments.
We keep tight control of our budget, both in spending and tracking and are a bit "anal" to ensure we track and more importantly understand our "numbers".
Being that I retired a bit over five years ago (DW a few months ago), our personal rate of inflation (PROI) has actually been less than the stated current average, due to our personal lifestyle.
One thing to remember on any calculator is that the rate of inflation is constant, based upon long term history - as is any results, and most (including FIDO's RIP) can be overridden if you want to be more conserative.
Another is that for us (being retired), our starting number (along with inflation rates) are based upon the current day's number. Sure, if you use any calculator as a "guesstimate" for planning during your accumulation years you may be off over the long term, but our long term grows shorter and shorter with each passing year, being in the distribution phase
thus being a bit more accurate in the current day, and becoming more acurate with each passing year.
BTW, year over year of the RIP forecast during the last five years of retirement have actually resulted in being better than plan. That's due a bit to RIP discounting your first year retirement assets by a bit more than 10%, to account for the possibility that you retire in a down market situation. Unfortunately, a lot of folks can't choose the date of retirement, but the tool takes that into consideration when running the forecast.
Is it (or any software forecast planning tool, paid for or free) is not perfect. However, it's a bit better than nothing or even a spreadsheet which cannot take advantage of the various simulations that are run against long term market conditions - both positive and negative. And in RIP's case, it also interfaces to M* to break down your/spouses current day portfolio (both FIDO and non-FIDO holdings) to see the actual breakdown of your holdings as input to the forecast model.
BTW, in another post (and not a personal "dig" - your life is your life) you stated that you trade about 1-2x a week. That means you change your portfolio mix (regardless however small) more than 50-100 times a year. I would think that would add a bit more varience, regardless of any plan you are trying to project. For me, that would make my personal plan (regardless of forecast tool) "suspect", regardless of any projected return or anticipated inflation rate.