Well I'm not sure about the ratio, but here is some science and hopefully the answer to your question.
The goal for you is to develop a mash that can be converted as much as possible into fermentable sugars. Typical mashes leave a big chunk of unfermentable sugars in the wort which add residual sweetness which is good for flavor, but bad for t3h diabeetus!
In order to achieve this you will want to mash at 149 degrees. This maximises the diastatic power of the beta-amylase, meaning the enzymes break down both carbs and complex sugars (unfermentable and bad for your condition!) into simple sugars which the yeasties will consume so you don't have to! Beta-amylase activity peaks at 149 so it's critical you mash at exactly that. Alpha-amylase does help because it produces the complex sugars for the beta-amylase to convert, and starts working its magic at 140ish but peaks way higher, so even tho you will get a little activity at 149, it's worth the tiny sacrifice to ramp up the beta-amylase. In order to make sure you get all of the conversion into fermentable sugars possible it is recommended to add alpha-amylase enzymes to your mash manually because your mash temp won't be high enough to naturally get them working. This combination of the perfect beta temp and the addition of alpha will convert the most sugar possible.
After the saccharification rest of a minimum of 45 minutes, mash out to 172 degrees. The temps of your mash will blow past the maximum limit of beta-amylase and near the working limit of alpha-amylase virtually stopping all starch conversion. Now it's all up to the yeast. You'll want to probaby do a lager yeast if possible since they will consume not only mono and disaccharides (like ales yeasts do) but also trisaccharides (which ale yeasties do NOT eat) resulting in less redidual sugars in the final beer.
Since you'd be doing a lager I suppose the you'd wanna pick from the available styles to whichever you enjoy drinking most. I'm assuming it's better to pick a yeast that ferments a bit on the dry end so you get more alcohol and less sugar in the final beer. There is a pretty sweet article on this, from which most of this information was originally taken at this link:
http://www.bacchus-barleycorn.com/PDFfiles/LearnMore/Beer Lowcarb beer.pdf
Hope this helps.