I spotted this solution at The Tennis Show in NYC earlier this fall.
Rather than measuring and fussing with a rusty, slipping buckle,The Player Strap offers a portable tennis net correction tool that goes on quickly and firmly over an existing net strap insuring your game's net is set at 36." It's best for solving the too high problem, and yes, a net that is even one inch off makes a difference, especially to your serve.
Handy add for your tennis bag, $12.99 USD.
Find it at Tennis Solutions.