Horse bows weight-wise are tuned for their default draw. So as an example, a korean bow that is #40 will be tuned @32". If you shoot it mediterranean with 3-fingers @28" it will have less. So a #45@32" korean bow could have... let's say around #37@28". There's no rule really.

Also, I would highly recommend not going further than #30 with your first thumb draw bow. It's like with any beginner within a style. Your thumb is not used to such strain and might get injured. Keep in mind that whatever you shoot normally with three fingers you will now shoot with one.

Arabic bows are nice for first thumb draw shooting since they are short and and their draw is pretty much the same as if you'd shoot them western-style. So for example, a turkish bow is 28" and you can shoot both styles with the same poundage and arrows. The Alibow arabics are also cheap so if you'd want to progress higher and get a new, strogner bow, you did not pay much to get to this point.