Knots to MPH & Airspeed
Don't concern yourself too much with the difference between mph and knots. The 15% conversion won't make much difference. 1 knot ~ 1.15 mph or 1 mph is about 87% of one knot... so 20 knots ~ 23 mph, but keep in mind that your true airspeed increases inversely with the square root of the change in the density (altitude), so on launch at Pine Mountain SS your true airspeed is 1.15 times what it was at sea level.
Our flight characteristics are so erratic that 15% of anything doesn't make much difference.
Indicated airspeed is a differential measurement between dynamic pressure and static pressure. Your indicated airspeed varies with density such that the indicated airspeed that rigid wing flight characteristics occur at will be the same regardless of changes in altitude and density. Example; the stall speed will be the same indicated airspeed at sea level and high altitude. Rigid wing aircraft reference indicated speed from a pitot tube for flight instruments because their flight characteristics are more closely linked to indicated airspeed than true airspeed. They use true airspeed for navigation.
Propeller driven wind meters and GPS receivers typically display true airspeed. The low dollar Hall wind meter is somewhere in between. It uses the difference between dynamic and static pressure, but it is also using gravity and a tapered gap rather than a bellows and spring. If the wind was smooth, you could mathematically handle 15% (note) more at Pine than you could at Bates, but Pine will be more turb so probably not.
Note: the air at 7,500 feet is about 3/4 as dense as it is at sea level, so your true airspeeds will be about 15% faster at Pine Mountain launch altitude compared to seal level airspeeds, and higher as you gain altitude. To calculate the conversion factor: take the square root of the reciprocal of 3/4 ~ 1.15 or about 15% more.