Due to the varying character space available in message previews for preheaders in different email clients, I was wondering is anyone stuck to any guidelines on a minimum/maximum character count.

Particularly for shorter preheaders, ensuring no other parts of HTML text like "web version" or alt text of a company logo appears in the preview, as these are the most common text parts to follow after the <body>.

I believe you can pad out the preheader with &nbsp; to avoid that problem, but I'm not sure how compatibile that is across email clients.

I have a minimum of 35 characters on the basis of Outlook clients, but wanted to get other developers thoughts on this.

I noticed there is a great guide from Litmus here: https://litmus.com/blog/the-ultimate-guide-to-preview-text-support but the clients that basically make the preheader variable based on the subject line make it difficult to determine an average character length.