John, may I ask the suggested value of the rug you are working on. If it was me I wouldnt apply any type of protection to it. Particularly if it is that deep a pile as unless you can apply the retarding to all of each fibre then it is a waste of time and you are doing an injustice to your client. You can apply it so that it adheres to all of each fibre but to do this it needs to be taken away to be done as it cant be done insitu. To do this to your rug would be £720:00, a reasonable "security" for your clients expensive textile.
Best, Dave.
P.S. I have to also say this and hope it helps.
There is never a good reason to apply protection or anything else for that matter at cost. We are in this business to provide a service to our clients and for that privilige they pay us for it. If you do it for free, half price, or cost do you still offer a call out service which should be standard practice after applying protection. If you have done it and not charged accordingly then each time you go back you are at a loss, again.
If the textile is worth a boat load of money as you have suggested then it should cost a boat load of money to clean, maintain or do whatever you need to do to it. More to the point if its worth a large sum then your client will want it maintained and now as is you are going to have to maintain it for free.