The easiest and most accurate way to rout the correct sized groove for intumescent strip is to use a router cutter designed specifically for the purpose.
We have a full range of these in the Craft Pro, Trade and Professional brackets to cover all the standard strip widths currently in use; 10mm, 15mm, 20mm for the Trade range in 1/2in shank, with the same shank and groove widths available in the Craft Pro range alongside a 10mm and 15mm width in 1/4in shank.
The Professional range are all 1/2in shank with the addition of a 25mm width cutter. Each cutter has a bearing guide on the shank that sets the correct 4mm depth for a perfect flush fit so all that is needed is to set the correct depth.
The cutter is fitted to the router paying attention the 'K' mark on the shank to ensure it is fitted correctly and tightened off.