England ended the third day with a lead of 186 runs for the loss of 7 wickets, but if they can add a few more runs in the first session of the fourth day, then they will definitely take the lead beyond 200 runs. In this case, if England can get out three or four batsmen of the India toppers while completing the lead naturally, then their chances of winning the match will definitely increase a lot. England are waiting for an opportunity where if they can bat better in the first innings and take it beyond 600 runs, then they will definitely go beyond 250 runs. England's chances of winning this Test are 80%.
You are right England start to Day 4 is key. If they can get their lead over 200 runs and quickly dismiss some of India main batsmen their chances of winning will greatly improve. It seems there was slight confusion in original text about England first innings. Real focus should be on how many more runs they can add to their current lead in their second innings. If they manage to build a lead of 200-250 runs and take early wickets they will be in very strong spot even if 80% winning chance might be bit high for Test cricket.