If I want to push the ISO on my FM2, what is the best setting?

There is no best setting, globally...there is only the best setting for the scene at hand. If you can get the aperture and shutter speed values that you like at ISO400 - then there are few reasons left to push the film1. If you need either faster shutter speeds or a more open aperture but can't because of your ISO, then you should start looking into pushing.

Pushing film has consequences, namely, increased contrast and grain and decreased shadow detail. The Kodak Fact Sheet for TMax 400 states:

Because of its great latitude, you can underexpose this film by one stop (at EI 800) and still obtain high quality with normal development in most developers. There will be no change in the grain in the final print, but there will be a slight loss of shadow detail and a reduction in printing contrast of about one-half paper grade.

When you need very high speed, you can expose T-MAX 400 Film at EI 1600 and increase the development time. With the longer development time, there will be an increase in contrast and graininess with additional loss of shadow detail, but negatives will still produce good prints. You can even expose this film at EI 3200 with a longer development time. Underexposing by three stops and using three-stop push-processing produces a further increase in contrast and graininess, and additional loss of shadow detail, but the results will be acceptable for some applications.

This resource also states:

Push processing allows film to be exposed at higher speeds, however, push processing will not produce optimum quality. There will be some loss in shadow detail, an increase in graininess, and an increase in contrast. The degree of these effects varies from slight to very significant depending on the amount of underexposure and push processing. The results are usually excellent with a 2-stop push, and acceptable with 3-stop push depending on the lighting and the scene contrast.

And down in the caveat-rich fine print (bolding mine):

For high-contrast scenes, such as spotlighted performers under harsh lighting, expose and process as indicated in the table. However, when detail in the deep-shadow areas is important to the scene, increase exposure by 2 stops and process your film normally.

Think about that for a sec - when shadow detail is important, increase exposure by 2 stops! This is the exact opposite of pushing the film.

---

In summary - there is no best setting, only the one you need to get your shot. However, each of these options comes with a tradeoff - and whether or not that tradeoff is worth it is really up to you. (As an example, I once pushed Delta3200 to 12800 for a high-school football game. The results were more akin to a bad halftone blown up than a photograph)

1: An increase in contrast isn't necessarily a bad thing. I live in the PacNW these days where overcast is the joy of life - so I find myself not really worrying about a little extra contrast. One may also push film simply because they like "the look"