good vacuum reading?

[rsmartin]

86 560SEL 96K

at idle in park @ approx 800 RPM vacuum reads about 17.5. actually it fluctuates up to 18 and down to 17 but mostly at 17.5

in D or R with parking brake set without foot on brake and RPM about 600 (hard to tell exact RPM by the dash guage) the vacuum drops to 15.5. same fluctuation up and down about .5

is that a good reading. is the two point drop when in gear normal?