Since there is no room to negotiate or even bypass the transfer, they are best used only in the case that game is certain and slam is not possible. Thus, after 1NT, a Texas transfer requires a 6-card major and between 10-15 points with no hope of slam. After 2NT, 5-10 points are needed.
Extended Texas transfer Edit
Extended Texas transfers are a variant to Texas transfers, enabling the use of Texas transfers after a negative reply to Stayman, showing 6-4 hands that game is certain and slam is not possible. After a negative response to Stayman, a bid of 4♦ or 4♥ is a transfer to 4♥ or 4♠, respectively.
If this convention is combined with Smolen transfers, there are two ways to show game-forcing 6-4 hands after a negative reply to Stayman: one is to apply an extended Texas transfer directly, the other is to apply a Smolen transfer, then re-transfer if no fit is found.