7mm is the most common line spacing for ruled notebooks. Sometimes I find this spacing a little too generous. Here is a survey of high-quality notebooks by line height.
N.B.: All of these have lined paper, unless noted otherwise.
Kokuyo Campus, Blue (A5 on JetPens)
Rhodia Unlimited and Webnotebooks
Midori Colors (JetPens)
Clairefontaine (1951 Collection on JetPens)