Fragment caching with touch (2.3.5)

I am working on an ecommerce shop. Currently we cache all the product
thumbs using fragment caching with the product key and we expire the
cache using Touch.

<% cache product do %>
<% end %>

Now if I want to cache the product page, what key do I have to choose
considering that I am already using the product key. And if I don't the
product how am going to be able to expire the cache with touch?