Editing Landsoftheblacksea:Main Page/house rules (section)

=== Movement & Spellcasting ===
Characters can move and cast up to a limit - take the spell casting time and subtract from 10, then divide result by 10 - characters can move up to this percent of their movement rate. Penalty is that characters will suffer an initiative penalty related to how much time they use moving before they cast.

Example: Gabriel is going to cast a spell with a casting time of 3; that means at most he can move (10 - 7) = 7/10 = 0.7 or 70% of his base move (8" = 80 feet per round right now due to light encumbrance). 0.7 * 80 feet per round = 56 feet. But in this case, Gabriel would suffer a +10 modifier to the initiative roll (+3 for the spellcasting, and +7 for using 70% of the round to move first).

If Gabriel decided not to move at all, his initiative modifier would just be the +3 for the spell.

If Gabriel decides to move 25 feet then cast, initiative modifier would be +3 for the spell, and an additional 25/56*7 = +3 for movement, total of +6.

If Gabriel casts and then moves, he can still move up to 56 feet after casting, his initiative modifier is just +3 for the spell; but we would only resolve his movement after the rest of combat happens (e.g., everyone else completes their actions for the round, and if he is still able, THEN he moves to where he wanted to).

I'll do the math for you if you want to do it, but above if how I've always handled movement in combat. 

I do similar things for melee & missile attacks - in melee, weapon speed takes the place of spell casting time (which means for weapons with a speed of 10 or more, you cannot both move and attack). Likewise, for missile attacks with a rate of fire of more than one, it's not usually possible to fire more than one shot and still move in a turn (since the second shot has a modifier of 2x weapon speed, 3rd shot has 3x weapon speed, etc. etc.).