moveq #0,d0
moveq #0,d1
move.b $300.s,d0
divu.w #$a,d0
move.b d0,d1
rol.l #4,d1
swap d0
add.b d0,d1
Converted value will be left in d1
I've never had to do it before, so forgive the non-optimisation!
Also, because its a byte conversion, its only good up to a hex value of $63 (99 in decimal). But it does as you requested