Steve Fosdick wrote:
If the text is always the same you could experiment manually with one image until you find the proportion you are looking for then divide the image width by the point size you finally settled on to arrive at a pixels/point factor.
Unfortunately the text will vary.
I thought I could be clever and use the imagettfbbox PHP-GD function to determine the size of the text for any given pointsize, and loop through pointsizes to find a suitable one. And that works incredibly fast, which surprised me. But sadly the result from the GD libraries gives the wrong results when applied to the ImageMagick programs.
Looking again through the ImageMagick docs I've found that various functions do exist for what I want, for example label: and caption: achieve the sizing I need, but I can't see how to actually use them for what I want so I've a dual pronged approach of experimenting wildly and asking on the ImageMagick forums to see which gets me a result first - but at this stage I'm pretty sure ImageMagick will be able to do what I need when I find out how.
I'll post the answer here for anyone who may be interested.