Count a ten-card hand after the discard
The club run, three eights, and heart run use nine separate cards. The remaining 7♦ counts as seven deadwood points. Under a standard ten-point knock limit, this hand is eligible to knock.
A card can belong to only one meld at a time. Do not count the same card in both a run and a set when working out your deadwood. Variants can change the knock limit; check the rules at your table.
Understand what the opponent can do
After an ordinary knock, the opponent can lay off eligible unmatched cards onto your melds. That can bring their deadwood down far enough to tie or beat yours and undercut you.
After gin, there is no layoff. That makes gin attractive, but waiting gives your opponent time to finish first. A low knock early in the hand is often worth considering when improvement needs a very specific card.
Make a decision from visible information
Watch which discards your opponent takes. If they have been collecting cards near one of your melds, a layoff may be possible. If your improvement cards have already appeared in the discard pile, waiting for them is less promising.
Avoid a rigid rule such as “always knock at seven.” Your deadwood, available improvement cards, the stage of the hand, and the match score all influence the choice.
Try the comparison
In the example, discarding the seven after drawing a fourth eight would leave all ten cards melded. But only the missing eight completes that particular route to gin. Compare the value of ending now with the uncertainty of waiting. Play a few CPU hands and notice how often an extra turn changes the result.