Thanks for the link to that map! Fab. Found Achilty - there it is Auchiltey - good enough! Nearer Buchlyvie. Will let you know if I find anything like Bohellochan.... afraid I'm not going for the Boquhan option! "Did I hear someone mention Milngavie"?
I have found Bohellachin on the Stirlingshire map from John Thomson's map of Scotland (1820) on the NAS digital library website. I would be very surprised if it is not the right place.
http://www.nls.uk/maps/atlas/thomson/453.html Drymen is the Y-shaped parish west of centre on this map. Click on it until you can read the placenames. Bohellachin can be found above the Y and M of DRYMEN (parish name, rather than town name). Fourth place name up from DRYMEN.
Bohellachin can also be found on the 1850s OS map http://www.nls.uk/maps/os/oneinch_1st_list.html : Sheet 38 - Loch Lomond. Click on the point midway between the centre of the map and the south-east corner. You will see the Loch Katrine Aqueduct crossing a road, forming an X-shape on the map. Bohellichan is found by that crossing.
On a modern map, it appears to be in the midst of the Queen Elizabeth Forest Park. It must be near Drymen Road Cottage. Go to www.streetmap.co.uk, and do a search for grid-ref NS506939. You will need to zoom out to see it on a meaningful map. or --> click here <--
A piece of trivia - a Google with the spelling 'Bohellachin' gets one hit. In a Google book - History of Stirlingshire by William Nimmo. It lists the meaning of Bohellachin as 'plain of wooded promontory' ...