You have to completely explain your idea about random digits- I have no idea what you mean - please explain. You have to realize, when I read that, I take it as : A) The user is typing something when they mine, or B) You are trying to hash suffixes in a block. I think we need the verbose description of the idea.
Sorry I am not explaining myself. You take the hash of the miner's reward address. The last number/letter of the hash has to match the block that will be generated. That gives you 1 in 16 chances that you mine the block.
On
Block 148755, the miner is BBpD9ZBUQVh2SUVWnAsYY8XtPKfZBtvL7b
The SHA256 of that address is 97a86de22f791f1a95df46b560ca09f504cd3d607ea1473720563b340f370bbe .
The last letter in hash is e, so that candidate block hash also has to match.
If the block hash was 2852066294dd96f8e1709686dd5fdbf3501e1f9ef95b4e753d299660ec78e480 , the block would have been rejected under this rule.