a very rough guide is to add 3 percent for every rep you can do, for example,
you can bench 225 3 times, your approximate max is (3*3 percent + 100) * 225 = 1.09 * 225 = 245
you can bench 225 10 times, your approximate max is (3*10 percent + 100) * 225 = 1.30 * 225 = 293