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Why do you want to re proof a polyester flysheet?

If it leaks then there is a good chance it is one of the seams that is leaking and not the main flysheet

When a polyester starts to leak it is more or less the end of its life as the waterproof coating on the inside of the fabric starts to break down, this is often due to UV damage and reproofing may extend the life by a  another couple of seasons but when this happens it is probably time to buy a new tent

polyester is not a breathable fabric so your proofer should be OK but may not be necessary   

Is the tent actually leaking? Are you confusing condensation on the inside of the fly with rain leaking through? Most small tents will suffer with a bit of condensation as PU coated polyester is a non-breathable fabric, so the moisture in the air you breath out will form condensation on the PU coating.

As Force Ten says, it could also just be the seams which are leaking. If so, all you need to use is a seam sealer, which is quite a cheap fix.

If the fly is leaking, then it's due to the PU coating breaking down from many weeks of use. You can tell if this is happening by checking the inside of the fly to see whether the coating is slightly milky looking.

Before you spend time on re-proofing the tent, erect it in the garden and hose it down well whilst someone sits inside it to see whether any water is coming through. Don't point the hose straight at the tent, spray above the tent so that the water falls on it like rain. If no water comes through the fly, then it doesn't need waterproofing. 

if you look at tha back of the flysheet you will see the waterproof backing if this is cracked and danmaged then that would be why the flysheet is leaking

if the waterproof layer looks intact then there is a good chance it is a seem 

the spary you have bought will be fine as polyester is not a breathable fabric