bit of algy-bra
337x = 214
x = 214/337
x= 0.635
To deliver the same amount of fuel, a 337cc injector opens for 63.5% of the duty of a 214
However, when starting with the 214 map, you need to multiply by 0.635 which is the same as reducing it by 36.5%
If you think about it, it makes sense, you need to drop it by "about a third".
This doesn't take into account the injector on and off times, but the result will close enough to be safe to work with.
I see where you were going with the 55%/45% - you started with 214x = 337 - but you're talking about reducing your map by around half - that isn't right...