FR-4 has a dielectric strength of around 20KV/mm, so for a 1.6mm PCB, it can hold off 32KV without breakdown. 275V is nothing.
The requirements for mains spacing and pri-sec spacing vary depending on the intended application of your circuit, board contamination level, altitude, and peak working voltage.
The simplest way to comply with just about every safety standard is 8mm creepage/8mm clearance. The minimum clearance per IEC60950/62368 (ITE/ICT) is 4mm. But, if you want to use in altitudes up to 5000m, which is a typical requirement, you need to to multiply the clearance by 1.48...so the requirement becomes 5.92mm. The creepage value must equal or exceed the clearance value (by definition), so the creepage requirement also becomes 5.92mm. If this was for medical equipment, again the requirements vary by intended usage, but in general the creepage always needs to be 8mm, and the clearance can be 6.45mm (2xMOPP) or 7.40mm (2xMOOP).
This is why I just suggest to use 8mm/8mm to cover all possible requirements worldwide. Assuming this is just a project for yourself, you can safely go with 6mm/6mm.