The Interrail delay compensation scheme gives you €12 for a 60-119 min delay or €24 for 120+ mins. These values were standardised a year or two ago; previously the amounts were generally lower because they were based on a percentage of the deemed daily value of the pass.
Their process is quite straightforward and claims are generally assessed very quickly, usually a day or two of submission. Payment tends to take another week or so.
You're not doing anything legally wrong if you submit a Delay Repay claim to a TOC, but in my experience, Avanti are likely to deny your Delay Repay claim and direct you to claim via the Interrail scheme.