BIAB is a little bit different, and I haven't had a chance to play with some test mashes myself, but I do suspect that for Full Volume BIAB mashes (Where you have much higher liquor to grist ratios than trad 3V mashes) then you are going to need to reduce the alkalinity further than you would think.
Doing a series of test mashes is very simple Firstly decide what it is you want to test . . .Lets say it's alkalinity
Get hold of half a dozen 500ml pyrex beakers
Make up your grist and for example lets say 90% pale, 5% crystal , 4% Wheat and 1% Roast Barley . . .Remember you are only going to be using 125g at most in each beaker
Make up your water . . . you will need 6 different values lets go for 0, 25, 50 , 75, 100 and 150 . . . . Of course if you have more beakers you can extend the series, say to 200, 250, and 300mg/l . . . depending on your water make up of course you may not want or need to go this high. (While it may make sense to prepare this water using RO or deionised remember that using RO will dilute the calcium content as well so you should add calcium to compensate . . .Keep everything constant apart from the thing you are changing . . . so alter the liquor using acids, or the way you normally do)
Get your water to 'mash temp' and you may want to do this with the grain too, an oven set at 65C is useful for this. . . I use an deep oven tray containing water at 65C to act as a water bath.
Quickly mash in each of the mashes, and note the time you did so . . . Slap it in the water bath, remember to use the liquor to grist ratio that you use when you are mashing at your full volume. You may want to stir the mashes every minute or so.
After 10 minutes has elapsed for each mash, take a sample of the liquor, and cool it rapidly to 20C. ( I take the sample with a shot glass that has been in the freezer . . .instantly cools to 20C) and take a pH reading with an accurate, calibrated pH meter (Strips are a complete waste of time don't waste your time using them).
You can then plot the results of pH vs Alkalinity, and it will show you the best alkalinity to use for the grist you are planning.
Of course the next step is to go one further, and keep the grist and alkalinity constant (You determined the optimum one above
) and then vary the calcium content.
It's an interesting way to while away the hours of a rainy sunday waiting for the mash to complete, or the boil to finish or the wort to cool.