Careful using the M & Y values of the ASME code. First of all, you won't find any of today's gasket materials listed in the Code. More importantly, the M & Y factors are for vessel design purposes, and not assembly purposes. If you're designing a vessel, then ask the manufacturer of the specific gasket you intend to use for the materials M & Y factors. If you're looking to determine assembly gasket stress and bolt load, again check with the manufacturer but ask for the material's "minimum seating stress". The minimum seating stress (psi) times the gasket contact area will determine the minimum assembly preload. If your flanges, bolts and gasket will take it, try to achieve 50% yield stress in the bolts. This will develop greater elastic stretch in your fasteners.
Minimum seating stress values I use are as follows:
Flexible graphite filler, pipe flange spiral wound
= 10,000psi
Same, with inner ring = 15,000 psi
Elastomers = 500 psi (1,500 psi max)
Flexible graphite sheet gaskets = 5,000 psi (15,000 psi max)
Soft iron = 20,000 psi
Be careful not to exceed the maximum stresses on elastomers and flexible graphite sheet gaskets.