The excellent website changpuak.ch here and here allows you to figure out what core and number of turns for a given inductance, for ferrite and iron powder toroids respectively, and also to see the impact of adding or removing turns. It is really great.
I decided to create a table in Excel of the various ferrite and iron powder toroids, so that it is possible to type a desired inductance, and see the number of turns for all toroids at the same time, or to type in a number of turns and see the inductance for all toroids. Actually not all toroids (I only picked a selection), but it is useful for quickly evaluating which toroid size, and then using the changpuak website for fine-tuning. Another thing – Amidon’s ordering pages specify AL values which are slightly different to their PDF reference. I used the AL values from their ordering pages, whereas the changpuak site uses the PDF reference as far as I can tell. I don’t know which is more accurate.
The Excel spreadsheet can be downloaded here: amidon.zip. Type a value in the green cell and results will be shown in the red cells alongside.