Add games first
For 6–4, 3–6, 6–3, the winner has 15 games and the opponent 13. The actual margin is two games. Subtracting 2.5 leaves the favourite half a game behind; adding 2.5 to the opponent puts that selection ahead.
Keep the sign and participant
Player A −2.5 and player B −2.5 are different contracts. A feed should keep the participant, sign and numeric line with the selection. A decimal price without those labels is insufficient for a meaningful comparison.
Set handicap is different
A two-sets-to-one win has a one-set margin. It is not a two-game margin expressed another way. Keep games and sets separate in a notes sheet and use the dedicated calculator to test a completed score.