Create two new columns:
cnt_p_utg_raise_when_short: sum(if[tourney_holdem_hand_player_statistics.position = (tourney_holdem_hand_summary.cnt_players - 3) and tourney_holdem_hand_player_statistics.flg_p_first_raise and (tourney_holdem_hand_player_detail.amt_before / tourney_holdem_blinds.amt_bb) <= 7, 1, 0])
cnt_p_utg_open_opp_when_short: sum(if[tourney_holdem_hand_player_statistics.position = (tourney_holdem_hand_summary.cnt_players - 3) and tourney_holdem_hand_player_statistics.flg_p_open_opp and (tourney_holdem_hand_player_detail.amt_before / tourney_holdem_blinds.amt_bb) <= 7, 1, 0])
Put the former over the latter, and multiply by 100. This will give you the percentage of the time he raises UTG (regardless of table size) given that he has 7bb or less. For more on creating custom statistics please see the
Tutorial: Using Custom Statistics and Reports.