I hadn't looked at the lactic acid requirement but I believe 6 mL isn't going to be enough. You are mashing with 6 gal water with alkalinity of 64/50 = 1.28 mEq/L. Total alkalinity for the 6 gal is 6*3.785*1.28 = 29 mEq. To reach pH 5.4 (a reasonable pH for a Kölsch) you'll need acid to the extent of about 90% of that or 26.1 mEq from which you can deduct at most 6.5 and more probably about half that for the calcium in the treated water so let's say you need 20 mEq for the water. One mL of 88% lactic acid solution contains 1061 mg of the acid which is 11.77 mmol. Lactic acid is a weak acid so it only supplies 0.97 mEq protons per mmol of acid thus 1 mL of 88% solution gives you 11.44 mEq protons and you need 20/11.44 = 1.75 mL for the water alone.
To pH 5.4 Weyermann's pneumatic Pilsner malt requires 9.9 mEq/kg and to pH 5.3 it needs 15. Thus your base malt, if Weyermann's pneumatic Pils will need from 76.5 to 115.9 mEq for pH's between 5.4 and 5.3. That's 6.68 to 10.2 mL of the acid (which is actually a little weaker at 5.3 than it is at 5.4) for the base malt. Now, OTOH, if you used Weyermanns floor malted Pils as the base malt the proton requirements for 17 lbs go up to 117.5 mEq at pH 5.4 and 145.8 mEq at 5.3.
At this point we have to account for the wheat malt which I can't do because I don't have any measurements for it but it is clear that it will have a proton deficit too though a relatively small one as it is only 8.1% of the grist. And of course I don't know which base malt you are using either. Treating the wheat as the same as the base in terms of its acid requirement it appears that you would need from 9 (pneumatic, pH 5.4) to 15 (floor, pH 5.3) mL of 88% lactic acid. The sauermalz equivalents would be 0.68 lbs (3.5%) to 1.2lbs (6.1%).
It might also be of interest to predict what the pH would be if only 6 mL of acid were used. For the pneumatic malt that would be pH 5.49 which would be fine. For a more alkaline base malt it would be higher e.g. 5.64 for the Weyermann floor pils which while higher than desirable would still give a decent beer if not one as bright as we might like.
Because no spreadsheet or calculator can give you an accurate pH estimate unless the malts have been individually measured it is best to make a small test mash (the same mash you intend to use for the beer but scaled down to a pound or less of grist). If you don't do the test mash at least stand by with the pH meter and additional acid at the ready.
