At 45 refresh the villain should be able to take out a whole city block without breaking a sweat, so (to me) that seems extremely high. And (other than refinement), won't you run out of powers at some point? The scarecrow is listed at -25 refresh, and even the -19 refresh version would still be pretty tough to deal with, I think.
I like to go quantity over quality in occasions like this. Look at the battle at arctis tor, part of the group had to stay behind to keep the mooks (and maybe even some lieutenants) away from those that entered the tower. If you can create a similar scene, you can easily scale the final villain down without taking away the challenge. And mooks are easy to come by, so you can increase their numbers any time (or throw a lieutenant or two into the mix), if you want to increase the pressure on the team. I especially like to do it like this, because the pressure is distributed more evenly. If a 45 refresh character concentrates his fire on 1 character at 7 refresh, he is probably going to end up as a piece of charcoal on the floor without a chance. If you distribute it more equally, every character has a chance to shine and they won't have to take hits that are quite that hard.
I know that is not exactly checking your math, more like... proposing a different formula. Hope it helps anyway.