Finding public key out of ECDSA certificate
The 00 04 tells me, that I'm lucky and no point compression is used, which means, that the abscissa and ordinate Key are easy to find out.


Nope. The 00 tells you that no bits are unused from the bytes making up the ASN.1 BIT STRING, which is indicated by the first 03 valued byte.'

04 does of course indicate an uncompressed point, so your end result would still be correct.


  Because of my prime number (related to my prime256v1) I know, that length of X and Y Key is 32 octets. So I take the first 32 octets to form the X-Key and the last 32 to form the Y Key


Officially the order of the curve determines the key size. Often there is no difference though. The name of the curve also contains a strong hint about the key size. The public key is however within $0$ and $P - 1$ inclusive (see comment of Poncho below).


  Because my program requires 36 octets for X- and Y-Key, I add 4 leading zeros to get my result.


That should be OK, if the result is indeed interpreted as big endian (network order) value. It beats me why you need to add 4 additional zeros (to the left, I presume), but that might be a hardware implementation issue.