Go with Daabs calculator. Estimate 10% for a 60 minute boil off, 15% for a 90 minute. Aim for 2.5X water to grain ratio, check how much water you have left in the mash tun after run off (think mine was 1.2l) and add this. Here's a quick one :-
Enter Volume Required In Fermenter ie Brew Length e.g. 23L: 23L
Percentage Loss During Boil e.g. 15%: 10%
Enter estimated loss to hops and trub eg.3L: 3L
Total Grain Bill e.g. 5 kg: 5kg
Enter Dead Space in Mash Tun: 1.2L
Enter Water/Grain Ratio for the Mash in Liters, eg for a 2.5 : 1 ratio, enter 2.5: 2.5L/kg
Wort Required for the Boil: L28.9
Made up from two equal quantities of:14.5l liters of wort collected from the mash.
Total Quantity of Water Required For Batch #1: L20.7
Mash Volume: L12.5
Top Up With: L8.2
Water Required for Batch #2: L14.5
Soooo, mash in with 12.5l of water. I personally add this to an un-preheated mash tun (Thermos) at 85c and by the time I've thrown the grain bill in and stirred I'm damned close to 68c. Dozens of methods of doing it as I'm sure you are aware but that's mine. After 1 hour stir and check the temperature, you need it to be within 62c - 68c ideally. If it's 64c or less I would add 500ml to 1l of boiling water (depending on how low it's gone) to bring this back up a little and subtract the water you have used from your batch sparge #1 addition.
So, to batch sparge #1. You're aiming for 75c to 82c and I have my water temperature set at 85c to achieve this. Sparge one is to add 8.2l minus any additions you added after the 60 minute mash, if you added a litre then simply add 7.2 litres rather than 8.2. You may not get upto 75c with such a small addition, this has happened to me on many occasions now but don't worry, just try get it there if you can. Give it a good stir, put the lid on and leave for 10 minutes then take the first runnings into a jug and return and drain into the boiler.
Once totally drained (this can take some time, don't panic), add 14.5 litres at 85c, stir, give it 10 minutes, take off 1st runnings then drain. Above all, don't worry about it too much, whatever you do will likely work regardless.

I keep overshooting on quantities and my efficiency is still not as good as it could be but who cares, I'm still brewing great beer at cheap prices.
NB: The above is in my experience and in no way endorses a sound way of brewing, merely my amateur attempts so far.
