Ekka's right thats the standard that we work to, one foot of radius for every inch in diameter at breast height, with some adaptations for age, health and vitality here's a local council formula from Oz with those adaptations included (BTW UK Arborists will recognise this table as being plucked from their standard!)
Table.doc
What I would recommend is that if you're going to severe roots then yes do as Ekka suggests but go beyond the radius calculated for the critical root protection zone, as the fine feeder roots are present much further out and will greatly benefit from the TLC you could give them in other words to steal Ekkas diagram treat as much of the green areas (both drk and light) as you can...your tree will thank you for it.
