Block value from strength not calculating right?

According to my tooltip in game, I am getting 513 block value from 1047 strength (1047 / 2 = 523.5).

I then removed all gear and talents, reducing my base strength to 175 (I am an 80 draenei warrior).
The tooltip now showed a block value of 77 (175 / 2 = 87.5).

I equipped a +29 strength weapon which gave me 204 strength.
The tooltip now showed a block value of 92 (204 / 2 = 102).

If I subtract 21 from my strength values across the board I get:
154 / 2 = 77 block value
183 / 2 = 91.5 -> round up -> 92 block value
1026 / 2 = 513 block value.

Is this as designed or possibly a bug related to draenei base stats?

The equation for BV is:

BV_from_Gear + STR/2 - 10 = BV

So with 1047 STR and 0 BV from gear you should get:

0 + 1047/2 -10 = 513.5, which checks out.






EDIT: The old equation (TBC and before) was

BV_from_gear + STR/20 - 1

So you can hopefully see how the -10 got in the new one.

... ok after reading that my head hurts, no wonder i failed maths

Essentially, calculate BV as you think it would be calculated and then subtract 10 from that.