"Heavy" wepons

I am unclear as to how the pricing of the heavy trait works on weapons. Presumably there is an increase cost for making a weapon require more power to use.
For example, I wanted to make a Recoilless rifle. That’s a sub index squad support weapon capable of damaging most vehicles. It has a long range and is totally useless without a tripod.
The heavy assault gun is a good place to start, the damage values look appropriate. That’s 4 pts to start as a SSW, 8 to up the range and the tripod costs 4 points. I then drop the ammo two categories for 9 points. So I have a 7 pt weapon. I want to up the power requirement on the heavy from 4 to 8 to make the weapon totally useless except with the tripod.
So what is the final cost?

Heavy is an obstacle limitation witch is priced based on the disadvantage it gives you. But the disadvantage is based on the characters power. Do I calculate of the maximum possible (in witch case the base cost of 3 would represent a heavy 1). Do I assume some basic level of competence and work from there (the lowest requirement in the book is 4 so assume 3 power and calculate the penalty) in ether case the cost of the weapon above becomes -1 (1 in practice) because I am adding a +8OB. 
I can see just making 3 the benefit regardless of how heavy the weapon is but that means buying a tripod mounted weapon is more expensive than an equivalent weapon without the wait problem.
Any heavy of less than 4 is meaningless so until I get a better proposal I will go with that as the minimum and 1 pt moves the power requirement 2 up or down.
This means that a recoilless rifle has a rb of 5.

The point cost of Heavy is worked simply off the Obstacle guidelines. As for how to set the Power level, I would use the General guidelines to cover it changes to the base cost.