That's probably an OK assumption - that is until you get to the length of the steering arms which become part of the equation to compute the actual ratio and then becomes steering effort. By using the turn plates you can figure your exact (or at least pretty close) ratio.

Short steering arms = tougher steering, long - you know the rest.

I would look for 4 turns or more lock to lock for non assisted steering (just found out mine is 14:1 - curiosity while answering your post)