A common misconception I see about 5-hit builds among some people is that as long as you are getting 20 TP/hit in your TP phase (which it seems like you are doing since with your proposed 81 STP you would be getting exactly 20 TP/hit), you have a 5-hit. However, unless you are WSing in the same amount of STP you are TPing in (you shouldn't be), this is not the case.
A Masamune's weapon skill set generally has 15-16 STP, depending on whether or not you have access to Tenryu Hakama +1. For those without, your WS set should be containing 15 STP (Rose Strap, Rajas Ring, Brutal Earring, Saotome Sune-ate +2)
I don't know how to explain things as well as a lot of people on this site, but I'll try:
11.1 * 1.55 = 17.2(floored)
100 - 17.2 = 82.8
82.8 / 4 = 20.7
So in this case, you'll want to shoot for 20.7 TP/hit in your TP phase to make up for the less STP you have in your weapon skill set compared to your TP set. This comes out to needing at least 87 STP including merits in your TP phase.
As for your question about 2-hit weapon skills, the extra hit is 1.[TP / 10(floored)]. For example, 50 STP including merits in your WS gear would result in 1.5 TP for extra hits. 60 STP would result in 1.6 TP for extra hits. 65 STP would result in 1.6 TP, and so on.
If there's something confusing in there let me know. Like i said I suck at explaining stuff. ;;