Bagpuss
Couple of comments: first, I think you can move him over to JWB faster than over a two week period without ill-effects - a week should be fine. Secondly, I'd go with the JWB recommendation on amounts if you think that he's too chubby at 15kg. In terms of how you tell if he's the right weight, there are two tests.
First one - get him to stand straight (i.e. no curve in his body which would overstretch the long side or "scrunch up" the shorter side of the curve) and run your hands lightly over his rib cage, then do it more firmly but without pressing your hands into his flank. If you can feel his ribs, but can't see them, and they only have a light covering of flesh, skin & fur, then that's a good sign. If you have to apply pressure to feel his ribs as you run your hands over them because he's a bit too "well-upholstered" - then he's carrying too much weight.
Second one - again, while he's standing straight, get behind him and look down at him. He should have a "rounding out" from his shoulders where his rib cage is, then it should taper in at the base of his rib cage so that, looked at from above, he has a well-defined waist between his rib cage and his haunches. No waist or chunky waist: less JWB for Charlie Chubs.
If he looks good and ribs feel good as he is - I'd still serve less JWB than Bakers to start with - maybe go for half way between the two amounts. Once he's 100% on JWB, you'll see within five to seven days if he's putting on too much weight and will be able to adjust downwards accordingly without it being a shock to his system.
Good luck and keep us posted.
D
