It helps to use brewing software. Brewer's Friend, Brew Pal, BeerSmith, et cetera; they offer scaling and then you can additionally adjust ratios and hop additions for what ingredients you have available.
I'm not a luddite by any stretch of the definition, but I resisted using brewing software for a long
time... but the right application really does help a lot. I was able to try them all
(since a Mac will run both Windows and MacOS with equal power and minimal fuss) and can only say, as I have in previous postings, that if you are considering a software solution and happen to be running MacOS, it really doesn't get any better than Beer Alchemy
These sorts of brewing apps generally have similar degrees of accuracy with regard to calculations and the like, but off all of them, Beer Alchemy has the best and most intuitive and friendly user interface of them all. And I find the accuracy of it's various calculations to be amazingly spot-on.
I'm not connected in any way with the developers of the application...just a very satisfied user, for almost five years (after having tried all
of the others).