Approach from trot - about 9ft (3 strides depending on the length of your legs
)
Approach from canter - assuming a canter stride length of 12 ft...then about 18ft minimum from the fence itself...to give a reasonable take off...although you might find your 17hh horse needs a longer distance, closer to 21 ft.
To be honest...I wouldn't bother with placing poles...if your horse needs help do a grid instead and change it for each horse if you have a helper.
S
You're right. it's about 3 strides from the fence.
It doesn't matter the height of the fence or the stride length of the horse, the placing pole remains in the same place. The horse should be taking off about 6' from the fence.
When a course of jumps is put up they work on a horse stride being about 12 feet & set it accordingly. They make no allowance for a horse that has either a shorter or longer stride. This is why you need to be able to vary the length of your horses stride within that pace.
Well think we're just having bit of a change tonight putting few jumps up and wanted placing poles as both horses can rush a little, did gridwork last time we jumped so will do more next time after tonight. Would eventually be nice to build up to maybe a placing pole and 2 canter poles before that to keep them steady