This is original code from my game. Driver for amiga and ST mouse.
You must call this procedure all the time. After every long break you must
call the init procedure. (Because the synchronization....)
Main proc:

 lda $d300
 lsr @
 lsr @
 lsr @
 lsr @
 pha
 and #10  (#3 in ST)
; there you check left-right move
 ldy #3
l1 cmp htab,y
 beq l2
 dey
 bne l1
l2 tya
 clc
 adc #1
 and #3
 cmp xind
 bne l3
 sty xind
 dec xcur
l3 tya
 sec
 sbc #1
 and #3
 cmp xind
 bne l4
 sty xind
 inc xcur
l4 pla
 and #5 (#12 in ST)

and there is routine for up-down. The same as left-right but:
xind is translated to yind
xcur to ycur
htab to vtab

Init proc: there you are only check actual mouse position
 lda $d300
 lsr @
 lsr @
 lsr @
 lsr @
 pha
 and #10  (#3 in ST)
 ldy #3
i1 cmp htab,y
 beq i2
 dey
 bne i1
i2 sty xind
 pla
 and #5 (#12 in ST)
 ldy #3
i3 cmp vtab,y
 beq i4
 dey
 bne i3
i4 sty yind

simpe,isn't it? 
So, for now only left tables:
htab:
Amiga: 0,2,10,8
ST: 0,2,3,1
vtab:
Amiga: 0,1,5,4
ST: 0,8,12,4
I hope it will works. As I know, Vblank interrupt is fast enough, so
after it, you needn't call the init proc.