The Middleham trainer favours a move up to two miles for last year’s Derby runner-up, in the Group Three race which is one of eight qualifying races for the bonus.
Last year the seven-figure pot was won by the John Gosden-trained Stradivarius, who landed the Yorkshire Cup before winning the Ascot Gold Cup, Goodwood Cup and Lonsdale Cup
Johnston said: “Dee Ex Bee has got entries all over the place, but we are probably going to go to the Sagaro Stakes at Ascot next month with him.
“We are looking at Ascot, because there is the small matter of the £1 million bonus potential on offer.
“He has come on great through the winter.”