Hey guys - resolution to your issues is simple trigonometry of arcs...

You don't even need to do the maths... (you just gotta know and when to divide or multiply by 2)

https://handymath.com/cgi-bin/arc18.cgiEnter into "Radius of Arc" (set units @ mm) 85 (i.e. Stroke spec. of NH 350)

& enter into "Height of Arc" (set units @ in) 0.5 (i.e Radovan's 1/2 inch)

Press "Calculate"

Result gives "Angle Subtended by Arc" (set units @ deg) = 63.45 degrees

The Angle in question is HALF of the Subtended = 31.725 degrees

To obtain the distance BTDC for the implied set 38 degrees: -

Enter 85 (units = mm) (stroke of NH 350) into "Radius of Arc"

& enter 2 x 38 = 76 into "Angle Subtended by Arc" (set units @ deg)

& set Units "Height of Arc" @ in/mm as desired.

Press "Calculate"

Result gives distance of 0.71" or 18.02mm BTDC to give 38 degrees advance at specified 85mm Stroke

Personally, I tweak my ignition timing by ear & feel...

Cheers, John

Please note; - the reason we have to multiply or divide by 2 - is that the Arc Calculator as linked considers the "Arc" to cover BOTH sides of TDC.