Remember, this spreadsheet is all about arm waving. You use the Color of the final beer to estimate how much alkalinity is required for the mash pH. Stronger beers of the same color would have more aciidity and therefore coul tolerate a higher RA, than a weaker beer.
I use the Morey calculation for color btw.
Using the weighted average of Lovibond of the malts is not recommended because of the way that SRM is actually measured. Dark beers have to be heavily diluted to allow light to pass thru them for the spectrophotometric test. Again, I use the Morey method. I guess I should have mentioned that in the Instructions...
Personally, I use the wankelman calcualtions for this.