Best heat pad size for a given build plate?

[First Layer]

So, I am building a Tiny M with several Mods. My build plate is a reused one from another printer. It is a MIC6 plate at 180mm. I had a 170mm heat pad on it, but it was just too big and left no room for the adjusters. I have a 120mm pad on hand. Will that be big enough or would I be better off with a 150m? 

What is the correct pad to plate ratio?

[mvdveer]

2 hours ago, First Layer said:

What is the correct pad to plate ratio?

I personally think that the more the plate is covered by the heatpad, the better as the heat distribution will be more even. (All parts of the plate will heat at about the same rate.) Having a smaller heatpad may cause the edges of the plate to take longer to come to temperature. If you do a heat soak on the bed, this should not really make that much of a difference., except time.

Don know the ratio - just my thinking.