Can anyone tell me at what percentage should the spar be placed for the horizontal and vertical . Is there a value to it??

It all depends on the stiffness of stabs, If they are good enough no need of spar.

Its a balance between weight and stiffness.

25 - 30 % of chord is where you would place the major spar on any flying surface. You can reduce weight by using sub spars ahead and behind the main spar and reducing the size of main spar.
Entire aircraft design is a game of balancing weight and required strength

Well..... adding to what the Flyingboxcar stated..... it needs to be such that it provides good rigidity to the structure.  The structure ideally should not flex or twist.

Flex is not a problem in itself. In fact high aspect ratio wings flex quite a bit.

Quote from: Naresh9807 on October 03, 2018, 01:35:34 AM
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

You asked for a tail spar, so here's what I would ask myself

1. Why do I need the spar? To provide rigidity
2. Why do I need rigidity? To avoid flutter
3. Why would the surface flutter? Because it will be subjected to flanking forces of air
4. Where would the surface get affected the most? Where there is the largest possible surface that comes in contact with air
5. What part of the largest contact surface be affected the most? where it is tethered the least and provides a longer lever arm
6. But if I add a spar (support in your case), will it tip the cg off. The tails forms a long lever arm with the fulcrum CG so a little weight change can cause significant imbalance.

Designing a scratch build is like a game of chess really, you have to consider state of the complete board, not a few pieces, but yes the first move decides the win