These are TDOC policies and subject to change.
Essentially an 8 year sentence stacked with a 3 year sentence, both at 30% are an effective 11 year sentence at 30%, this means after he serves 30% of those 11 years, he will then get a Release Eligibility Date (RED) to be considered for parole. To "flatten" his sentence he has to serve 7 months for every 12 months of his sentence, I believe this is called Determinitive Release.
On an 11 year sentence, his RED would be 3.3 years (39.6 months) and his "flatten" time would be 6.42 years (77 months)
But, another variable would be the underlying offense, some offenses are "no parole" or other percentages.