Can we estimate adult weight at 6 months

[dumbbird7]

Jack is now half a year old, born 15 June this year.  He is tall, very leggy, and weighs 18 k, or 39.50lbs, and pretty solid, much muscle. His dad was a tall slim BC, mum a Bearded collie..

Can we guess how big he will be when he is, say 18 months and finished growing?  He currently measures about 21inches at the shoulder, though measuring is a little tricky as the little blighter refuses to stand straight and still for more than a few seconds.

He loves to play with other dogs when we're out in the fields, and seems to choose dogs of his own size and weight but I worry a little about those Bambi legs when I watch the racing and rough and tumble.  After 15 minutes or so, I have to step in and spoil the fun, although he has so much energy, it's difficult to find a calmer way to allow him to let off steam without doing any damage.

We have a very small patio garden and very small bungalow (so no stairs to worry about thank heavens with regards to legs).  But if I reduce his play time he's hovering at 50,000 feet and we're scraping him off the ceilings! We do half hour early morning running loose in the orchard (no jumping), an hour mid afternoon mix of long leash, free run, perhaps other dog or two, and late afternoon another half hour. 2 hours altogether most days.  Difficult to keep him in check. Doing mind games, go find, bring me...doing what I call roadwork, leash training, wait, stop, cross over, keep in and so on, doesn't seem to reduce his energy levels.  Whatever he's having for dinner, I could do with some...Is 2 hours too long?  My old collie took to swimming in rivers when he was 5 months, any depth, any current, loved it.  He introduced the pup to his favourite stream several times before his final days and the passion has taken hold. I always felt water is kinder to joints and tires them out, but weather here is too chilly right now, though he makes a bee line for any river or little stream he spies, to go in knee deep, so perhaps this will be the way to tire him out when the weather warms up.