I figured out how to do this.
Change the step value in line 5 to make the spiral tighter (.2) or wider (.05).
0 'SPIRAL.BAS 5 ST=.1 10 PMODE 4,1:PCLS:SCREEN 1,1 20 FOR R=1 TO 165 30 CIRCLE(128,96),R,1,1,S,S+ST 40 S=S+ST:IF S>1 THEN S=S-1 50 NEXT 999 GOTO 999
As an experiment, I switched it to PMODE 0 so there could be eight graphics pages of spirals which could be page flipped to make a cool hypnotic animation.
I miss the fun of BASIC.