Riker, I have no idea what the ratio of 757 to 762/3 time is, it varies from season to season and base to base. I prefer the 763 as it is fitted with headrests and we get proper sized cups of coffee.
For my current roster I have no trips scheduled at all - I'm on reserve. Long haul would probably involve 1/2 or maybe 3 trips per month with durations of 3-7 days, the rest of your flying should then be short-medium haul with a 4 on/3 off pattern although the new Carmen process is changing this at the moment for some (not all are being badly affected by Carmen).