Yes, Cappelli is basically right (which is perhaps not surprising, he being an authoritative figure in the field of palaeography).
Every third word is probably above average, but in most documents one seldom finds a sentence without any abbreviations. Parchment was not cheap, nor was scribal labour. The same applies to manuscripts in any European language, at least in my experience (mostly Latin and Dutch). Some genres are more likely to contain fewer abbreviations than others, such as literary works written in book script; but even an illustrated Bible will usually be full of them. A few arbitrary examples, with abbreviations in bold:
http://www.sothebys.com/content/dam/stb/lots/L12/L12242/009L12242_6CB6T.jpg
Regnante domino nostro ihsu xpisto cum post passionem suam resurrectionem et ascensionem eius conuersis ad dominum cunctis finibus terre nec in angulo occeani aliquis se a colore fidei absconderet in conveniendo populos in unum et reges ut servirent domino. fuit in britanni(a)e partibus rex quidam deo notus tam uita quam nomine qui religiosus...
That's 32% of words.
http://www.villa.culture.fr/mediatheque/communs/images/grandFormat/0/21_2.jpg
Pars est prima prudencie: ipsam cui precepturus sis estimare personam. Neque enim formator agricole debet artibus et eloquencia rethores emulari quod a plerisque factum est qui dum diserte loquntur rusticis assecuti sunt, ut eorum doctrina non a dis?tissimis possit intelligi. Sed nos recidamus prefationis moram: ne quos reprehendimus imitemur. ...
That makes 43%.
On the other hand, there are also manuscripts that use few abbreviations:
http://www.europeanaregia.eu/sites/www.europeanaregia.eu/files/manuscripts/regiacarol_lat_5763.jpg
Gallia est omnis diuisa in partes tres, quarum unam incolunt belgae, aliam aquitani, tertiam qui ipsorum lingua celtae, Nostra galli appellantur. Hi omnes lingua institutis legibus inter se differunt. Gallos ab aquitanis garunna flumen a belgis matrona et sequana diuidit. Horum omnium fortissimi sunt belgae propter ea quod a cultu atque humanitate provinciae longissime absunt minimeque ad eos mercatores saepe commeant atque ea quae ad effeminandos animos pertinent inportant proximique sunt germanis qui trans renum incolunt quibuscum continenter bellum gerunt ...
That's 11%.
The overall ratio is difficult to establish (and perhaps less necessary), but you can get a fair impression by looking at a couple of documents in Google Images.