If I enter Thailand on a 90 day single entry non O retirement visa and I leave for a few days with a reentry permit, When I return do I get the balance of the 90 days from my original entry date, or from my reentry date please?

When you enter the first time, using your visa, you will get an arrival stamp, which will have an "admitted until" date.

When you enter the second time, using your re-entry permit, you will simply be given the same "admitted until" date again. In other words, a re-entry permit preserves the permission to stay that you already have been given.

OP, spot on answer from Caldera.

Note: you obtain the reentry permit from immigration office or airport on day of departure.

One convenient thing with airport is for optional 200b service fee they do the form photo etc. You just hand over the pp.

1000b + 200b

11 hours ago, Caldera said:

When you enter the first time, using your visa, you will get an arrival stamp, which will have an "admitted until" date.

When you enter the second time, using your re-entry permit, you will simply be given the same "admitted until" date again. In other words, a re-entry permit preserves the permission to stay that you already have been given.

Good concise explanation @Caldera