Hi again Mareid. I don’t think you have an older edition, it just takes a while to become familiar with vast amounts of information given in The Cake Bible….
If you look at page 231 (small print in the margin under classic buttercream variations) you’ll see that all the variations listed on pages 231-234 are for one recipe of either Classic or Neoclassic Buttercream. The first paragraph goes on to state that one recipe of each buttercream can accommodate up to as much as 1/2 c of liquid without becoming too soft.
Rose suggested decreasing the water in the lemon version of the Classic BC because it already contains liquid; for the Neoclassic BC you would just simply add the suggested fresh lemon juice and lemon extract in the same quantity suggested for the Classic BC.
Rose does extensive testing and has most likely tried every combination possible - she states “for a truly lemon flavor it is necessary to use both fresh lemon juice and lemon extract”.
Janis’s suggestion to add lemon juice in place of the optional liqueur is fine, but be careful - the recipe calls for 2-4 Tablespoons of liqueur; Rose suggests adding only 2 Tablespoons of lemon juice (plus the extract of course). 4 Tablespoons of lemon juice will be too tart. 2-4 Tablespoons of lemon liqueur would be okay though.
I hope that helps!