Total ignition advance??

[Fostang]

How do I go about setting the total ignition advance? The hormonic balancer can't read that high so how do I estimate it? I wan't to set it back at 38 degrees with base at 12. Can anyone help me out on the steps for accomplishing this. I know how to set the base timing but how do I know when the total timing is at 38 without reving the engine?

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67'fastback-Edelbrock carb,intake,cam,valve springs.Rhode lifters, CompCam rocker arms, windage tray,cerama coated headers,dual flows with H-pipe, modded c-4,tci stall converter,perma cool electric fan,aluminum water pump,griffen aluminum radiator,accel 8.8 cable,wide cap distriburator,magnetic pick up,v-matic hurst shifter,edelbrock performer nitrous, carter electric fuel pump,ignitor coil,solid motor mounts,magna racing suspension,poly urathane bushings,under rider traction bars, comp. eng. s