Can anyone tell me at what percentage should the spar be placed for the horizontal and vertical . Is there a value to it??
I don’t know whether you asked for theoretical reasons, or for something you are building.
The answer is that it depends on what you are building (besides the 25-33% chord guideline)
Eg.,
1. A 9-10” stab in a 30-40” span electric model in 2mm balsa or 5mm depron won’t need a spar.
2. For 10-15” stab, 3mm balsa will not need a spar. If 1.5-2mm balsa or 3-6mm depron, you could use a 10” bamboo skewer.
3. Larger sizes may need a 2-3mm CF tube.
Remember that we don’t know what size or material you are considering.
If your query is more specific, you’ll get more meaningful answers.
Besides size, you’ll have to say whether balsa or depron, sheet or built up, proposed covering, electric or glow, trainer or 3d etc...
As I said, it depends
Best wishes