Hi Spaceghost,
Congratulations on being inside the initial limits - here's hoping that the CAA medical chap measures you at the same level. In reality, I would imagine a degree of leeway is applied due to inaccuracy of measurement so even though you're on the border, you should be fine. Given your age, your astigmatism should now be static for the rest of your life and even if it does increase slightly,you should remain within renewal limits.
Good luck,
waterpau