Looking at the information that is available, I note the following:

Ilford gives times (4 minutes) for the films in T-Max RS at 24C.

Kodak gives times for a bunch of their films in both T-Max RS and X-Tol stock at 24C.

The Kodak time for the new T-Max 400 (TMY2) in T-Max RS at 24C is 4 1/2 minutes - which is close to 4 minutes - just a 12.5 % increase over the Ilford recommendation for their Pan films in T-Max RS at 24C.

The Kodak time for the new T-Max 400 (TMY2) in stock X-Tol at 24C is 5 1/4 minutes - which isn't far from 4 minutes - just a 16.7 % increase over the Ilford recommendation for their Pan films in T-Max RS at 24C.

So I would ask Customcolor to use their times for TMY-2 plus 16.6%.

You could do the same sort of comparison and extrapolation with other Kodak films, like Tri-X 400.