Would be great to be able to link the preview button to a specific file. E.g. index.html when working on

You can already do that: just enter the path of the local file in the address bar.

For example, if you are showing the preview windows for "js/script.js" enter "index.html" to preview the file index.html in the local root directory in Textastic.

You can also enter arbitrary URLs when you switch from local to remote preview.

Got it, great!

Thanks for a superfast reply :)