How do you keep rb300 steady while levelling it ?

If it swings back toward the arm rest, that is okay. Just make sure that it is remaining level as it moves. It's not that big of a deal. If you are very slightly off level, it's not going to ruin anything. I just eyeball it myself. I don't use a level or anything like that on the tonearm. I simply put a little bubble level on the TT and level that. Then the arm will be level, when it looks level, by eye. Don't make things harder than they have to be. Just get it where it looks good, and go with it. You'll have to dial in the VTF by ear anyway.

rb300 even if set to zero antiskate and perfectly leveled should swing slowly back to armlift. dial an antiskate first a-little less than the cartridge tracking weight.
if you have suspended turntable and detachable armboard than you should check level on the platter AND armboard as well.

I just used a bottle of rat shack cleaning soluion to keep the arm from sliding back to it's holster. Then I bought an inexpensive digital scale at the local "head" shop to measure my tracking force, and turned the tracking force dial to "3". I found that the trackiing force dial on my RB300 was not even close to accurate. The digital scale only goes to the first decimal(tenths?), but seems more accurate than measuring overhang and then setting the force by the dial! i think the scale set me back $50.