Length field as ASCII number?

[Knaldgas]

Trying to learn Rammbock.
Example of a message in this protocol:

\x020000165{"telNo":241,"et":"190521_152059.933","src":"F01-1","dst":"MFS","func":"DATINF","dateTimeGMT":"190521_152059.933","msToLocalTime":3600000,"timeZone":"Europe/Berlin"}0779320932\x03

I've managed to define the protocol:

New protocol   SDXP
u8      STX    2
chars   7    length
chars   165   json
chars   10    CRC
u8      ETX    3
End protocol

But that only works for this particular message.

• The length is 7-character ASCII encoded integer, how can I use that for the json length (instead of the fixed 165)?
• Is there a way to automatically calculate/validate the CRC (also ASCII encoded integer)?
• Is this the right way to define this protocol?