Remember that unreasonably high PoS block trust value was the reason why we needed Sairon's fix.
I thing it should be higher than now (prevPoWBlockTrust+1), but less than 2*prevPoWBlockTrust.
If higher, then mischievous PoS miner can withold each block, wait for receival of two new PoW blocks and then broadcast his with certainty that it would kick out both of them. That by itself is a small weakness.
But PoW after PoS block should keep it's trust doubled as it is now. This amplifies previous PoS block value in more "democratic" way.

I have been thinking a lot about this and my conclusion is that the more we try to award particular block/combination, bigger the chance someone will abuse it in his favor.

Here is more completed new functionality:
http://yacoin.net/NewGetBlockTrust2.htm