Im not 100% but if I remember correctly it should be around .020 of an inch. I would look into changing out the points to the solid state ignition. I did that to my 88 last year and it is very nice. I think is was around $100 and took 30-45 minutes
Did think of changing but boat runs sweet as is with points. I've done 200Hrs on the current points without problems so decided to give the ignition system a going over more as preventitive maintenace. 0.20 sounds about right to me.