As described in the chapter 5, dietary protein is converted to glucose with 58% efficiency (8). This reflects the fact that over half of the AAs can be readily converted to glucose. While some AAs can be converted to ketones in the liver, this is not thought to contribute significantly to ketosis.
Reference 8 is:
Jungas RL, et al. « Quantitative analysis of amino acid oxidation and related gluconeogenesis in humans ». Physiol Rev. 1992 Apr;72(2):419-48.
As for how much protein you need, the question is if you’re aiming for optimal muscle growth or just enough to get decent gains. The sensible low-end side seems to be around 0.82 g/lb, see « The Myth of 1g/lb: Optimal Protein Intake for Bodybuilders », while the sensible high end seems to be around 1.5 g/lb, see Lyle’s « Protein Requirements for Strength and Power Athletes ».
In destructsean’s case 157 lbs of lean mass (185 lbs, 15% BF) the 0.82 number would put him at 129 grams of protein per day, while the 1.5 g/lb would put him at 236 grams (it’s « ideal weight » in most studies and not LBM, but most people seem to use LBM anyway).
I doubt the body will truly use all the protein in the 236 gram case, so if he’s aiming to stay in ketosis he should probably aim a bit lower.
From a technical point of view, his brain will use around 100 grams of glucose per day, he’ll get ~25 gram from food and another 20 grams from the glycerol backbone in the triglycerids from fat, so he’s got around 55 grams of glucose to « spare » before his body has enough glucose and ketone production stops. At a 58% conversion ratio that’s roughly 95 grams of protein above his structural needs.
The DRI’s lists the daily protein needs as 0.8 grams per kilo body weight for adult males, around 67 grams in destructsean’s case, which would put his upper recommended protein intake at around 162 grams per day (67 + 95) to avoid getting kicked out of ketosis. Gaining muscle would increase this a bit, as would using some of the converted glucose (or unconverted protein) during exercise (or storing it as muscle glycogen).
This is very rough, and a bit simplified (there’s more gluconeogenesis that just from protein, not just the brain uses glucose, there’s some glucose production from fat/acetone, etc.), but it seems like the protein intake shouldn’t be a big problem given that he’s getting muscle grown and he’s likely using some glucose during exercise.
Getting a blood ketone meter and just testing different protein intake would likely be a far more efficient way to determine a good intake.