The corner of an old credit card gives a reasonable bead, if you don't have the proper tool
That's what the sealant gun nozzle is for. Cut it off at the required length to get the fillet radius you need.
The problem with Sealing baths/shower trays
As long as there isn't movement from the Tray/bath this then breaks the seal eventually.
1. Use a flexible sealant, ie: silicone.
2. Make sure the bath/shower can't move.
When I installed my bath, the first thing I did was
not use the hardware provided, there was nothing to stop the bath flexing when you shift around in it. The rim of my bath rests on heavy duty aluminium angle supports all round the periphery, these are not fixed using rawlplug type fixings, but large hardwood plugs hammered into the wall. It hasn't shifted or leaked in over 40 years, and with no maintenance.