Please help: Difficulty - Hashrate calculation

[cryptodave]

is there a way to calculate is it possible/is it worth to mine solo for different altcoins, if you have the diff and your own hashrate?

Is there a general formula for all altcoins or does it refers to the algorithm of the coin?

For example:

Altcoin a.)
Diff: 5.59488805
My hashrate: 1.805Mh/s

Altcoin b.)
Diff: 0.12345678
My hashrate: 1.805Mh/s

Solo possible? Chance to find a block?

I read a lot but I don't get it. ;-)