some help on sprockets please

[CyberTron]

Hi!

I have driven my new TMS (QingQI) 200cc for around 1000km now and i am thinking of regearing it a little.

but how?

I have 15/48 on now.

15/48 = 3,2 STOCK
15/45 = 3,0
15/42 = 2,8

16/48 = 3,0
16/45 = 2,8
16/42 = 2,6

a little question, does it matter if i go two tooths down back or 1 up front?

Now since it is hard to get a rearsprocket for my bike (dr200's are not common in sweden and my bike is a copy of that)
i am thinking of buying a 16tooth front.

I have these speeds today with 15/48 (the rpm's are not exact but somewhere around that), what will my new rpm be if go for gearratio 3.0 instead of 3.2? anyone care to take a guess? or must I have a gear ratio of 2.8 for it to go smooth in 90/kmh my goal is to get 90km/h around 4000-5000 rpms.

100km/h = 7000rpm
90km/h = 6500rpm
70km/h = 5500rpm
50km/h = 4500rpm

sorry for the long post and for spelling/grammar.