Roger,
I don't know of any campsites in the area apart from Brownmuir, but being an agricultural area there are lots of farms which I am sure wouldn't object to someone setting up in the corner of a field. There are also lots of wild camping opportunities in Drumtochty and the Fetteresso.
What directions are you planning to be approaching from and heading for?
There used to be one of the caravan club 5 van sites near Cairnbeg but they tend to be a bit snobbish about tents and I'm not sure if it still operates.
My Missus is in the SWRI in the area and knows a lot of the farmer's wives. I'm sure we could find something for you in advance if that is what you want.

I did a quick recce this afternoon and it looks like Pitrennie Mill is still operating as a caravan club site https://www.caravanclub.co.uk/certificated-locations/scotland/aberdeenshire/laurencekirk/pitrennie-mill/
I don't know if they would allow tents, but it might be worth giving them a call.
Mains of Fordoun Farm, Laurencekirk, AB30 1ST Phone 01561 320270; 07790 655486

You are a star Brian. Many thanks.
Also sent you an email....if your Missus knows a farmer.....Roger

Roger, I know that you have e-mailed me but I'll give you an answer online for the benefit of anyone else planning routes. I refer you to the route through the Fetteresso that I put together last year. http://pub9.bravenet.com/forum/static/show.php?usernum=757451871&frmid=14&msgid=793604&cmd=show&cp=1
The actual route is at https://tinyurl.com/y92g36da

You are describing exiting the forest using The Builg Mount route, an old drovers road.
Just before Corsebauld you will we walking down the east bank of the Bervie water, you need to look for a rough track leaving the hardtop logging road close to the bridge over the Bervie at NO 71628 83473, the actual turnoff is marked by a waymark post at NO 71679 83484. For the first few hundred yards the bank is flat but a bit tufty and covered in bracken, which shouldn't be a problem in May,a challenger pitched there last year, but if you continue down to the gate that is the end of Forestry Commission land, the other side is a big grassy field that is only ever user for sheep grazing. The banks of the river are ideal for camping just the other side of the gate and you would be well away from the buildings at Corsebauld.

I forgot to mention this to Roger earlier...

For anyone expecting to cross the railway line near the Castleton Farm café north of Laurencekirk , the Abbeyton Bridge has been closed until Jan 2020


https://www.theconstructionindex.co.uk/news/view/fate-sealed-for-frail-aberdeenshire-bridge-threatening-main-rail-line

https://online.aberdeenshire.gov.uk/apps/roadworks/default.aspx