craigo, the real rule of thumb for this caper is to completely fill the mash tun (i.e. the kettle) at mashing, so take the grainbill for a particular brew length, reserve some strike water and then when the grain is in to just use it to fill the kettle to the brim. Around 5kg tops in a 19L kettle is my advice, less than that should be much easier to manage.
During the boil again you want to keep the kettle full as possible until about 20 minutes from the end. Using sparge liquor for this top up has it doing something constructive rather than just a plain water addition. Sparge with about 2L/ kg of grain, if you are lucky you run out of sparge liquor to add right at that 20 minute mark.
Precise volumes don't really matter, as you'll be diluting it later, and so long as your efficiency is decent (tip- set it low to begin with, 70% is a good starting point) you should come close to the mark.
You probably want to dilute the wort once it is in the fermenter, so which ever way you end up managing it prior to going in there is up to you, I guess you could dilute it beforehand though. One thing to remember is that you are diluting to a target specific gravity, not to a target volume, and its probably most reliable to do this once the wort has cooled.
90 minute mash is mostly just a personal preference, probably more useful with drier beers, but it will ensure your mash has a chance to perform. If you want to stop at 60 minutes, by all means do!
Hope this helps! :icon_cheers:
