Flying under Class B

What is the minimum legal distance (altitude) to fly VFR under a Class B
"wedding cake"? CLE has a 1900 ft floor at the north side by Lake Erie and I
have flown at 1800 to go around CLE. I was not talking to CLE so I don't know
if they objected. Today I was coming around CLE decending IFR. When in VMS I
cancelled IFR and remained on squak for advisories. The controller wanted me
at 3000 when under a 4000 ft designated altitude. Was he just being safe for
his own good or is 1000 ft suggested as the minimum buffer? I know, I should
have ask him but the frequency was pretty busy.

Chuck