Many POHs give the maneuvering speed at max gross weight, but the lighter the airplane , the lower Va. A general rule is to decrease the speed by 1/2 the percentage decrease in weight.
You can find Maneuvering Speed in the POH or on the aircraft’s Type Certificate Data Sheet. For my Cessna 210 it is 135 MPH (117 Knots) at 3,800 lbs. Flying alone with full tanks the weight is about 3,000 lbs. The percentage decrease is 800/3800 = 26%. Divide by 2 to get 13%. A 13% reduction is 117 MPH (102 Kts). Empty weight on the 210 is 2,300 lbs so Va when empty is reduced by 20% (1500/3800÷2) and is 108 a reduction of 27 MPH. Every 100 lb reduction from max gross weight is about 2 MPH reduction in Va.
For my Cherokee 140 maneuvering speed is 129 MPH (112 Knots) at 2,150 lbs. Flying alone, with tanks at the tabs, the weight is about 1,850 lbs. The percentage decrease is 300/2150 = 14%. Divide by 2 to get 7%. A 7% reduction is 120 MPH (104 Kts). That works out to about 3 mph per 100 lbs.
A general rule of thumb then is that, in normal operations, you should reduce Va by about 2-3 MPH for every 100 lbs below max gross weight.
If you can’t find the maneuvering speed, a general rule of thumb is that it is 1.7 times Vs1 (Clean stall speed). For the 210 Vs1 is 80 so Va is 136 MPH. For the Cherokee Vs1 is 64 so Va is 109 MPH. On most airplanes, this speed is around 6:30 on the airspeed indicator.