Ethernet frame header size of TAP device and apropriate MTU calculation

[SVM]

Hello, all! I have many questions about tap device architecture.

What is a right way to calc mtu on TAP device to avoid fragmentation on 
real eth device?

I suppose TAP MTU = 1500-8(UDP)-20(IP)-18(Ethernet) = 1454.
So I'd set 1454 for tap device: "ip link set mtu 1454 dev eth0"

I'm not shure about what is the exact size of ethernet frame header, 
which tap device use in switch mode?
Is there FCS field? - Sure it should be.
But is there InterFrame Space and Preamble in the "switch environment"?

And how this correlate with PMTU and PMTUDiscovery options?
Should I really manually change MTU for tap device?

I very appreciate your help.
Thanks!