A 40-7 win over Edinburgh in their last game at a snowy Vicarage Road confirmed Saracens place in the last eight for the second season running.
The hosts bagged five tries against Edinburgh, who bow out of the competition after six defeats, with the other scores coming from Matt Stevens, Charlie Hodgson and Chris Wyles.
McCall said: "It was great to see Ashy score a couple of tries and getting back to what he really does best.
"The second try was fantastic and I wish I could claim credit for thinking that up; Ashy and Owen (Farrell) just saw each other and they pulled it off.
"We can appreciate just how difficult it was out there and we said after the game that the conditions were as bad as it gets with the ball like a bar of soap.
"The key was being patient and to keep our composure, but also to control the game enough through position and with that the opportunities would come and then we would put them away.
"I get questioned about why I start Charlie (Hodgson) with Owen playing well last week but they combined really well."
After three Farrell penalties, Saracens scored their first try when Ashton touched down Richard Wigglesworth's kick ahead before Greig Tonks hit back for Edinburgh's solitary score.
Edinburgh head coach Michael Bradley said: "For their second try, the tee was on the pitch and if the tee's on the pitch they have to kick at goal.
"The only reasons the guy next to Farrell can be on the pitch are if there's an injury or he's bringing the tee out and I suspect that that shouldn't have been a try.
"It would have made our lives a bit easier as well because momentum is a big thing in rugby matches.
"In fairness, it would have been hard to deny Saracens either way because they were by far the better side and they deserved to get the bonus point.
"So you can argue the toss about their second try but they still scored five tries so they deserved their luck on the day."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.