Your theory and math look correct. With a 1.25" throw on the 3/4" master cyl your only going to see 3/4 of the resulting travel on the 1.0" slave. You'd need to get a 1.0" master to achieve the same travel at the fork. Which it looks like it would just make your numbers needed.
Using Wilwod masters as a base a 1.125" master would overdrive your slave a bit if you wanted to go that far and be safe.
Wilwood Disc Brakes - MasterCylinder No: 260-10376
Or drop the dia of the slave to 3/4" to maintain a 1:1 movement-reaction ratio with your current master.