I think 9th & 10th tans are just too far off for most clients to think about.
If you want to offer something off tans, then the 4/5th tan is more likely to get someone booking than something off 9/10th tan.
Why not do 25% off your 5th tan or a half price 5th tan. This way you are getting something for the appointment but the client is getting something too which will hopefully be enough to encourage them to continue to come to you.
I do 3 for 2 in the quieter months and take the full payment for the 3 tans on the first appointment and then do the 3 tans within a month (these can be 3 seperate tans or 2/3 people together). The clients love it as they feel they are getting something for nothing but I usually end up doing 3 people at the same place and time so I save on my petrol :biggrin:.