P.S.:
images for Sirloin Tip Roast :
so is your roast like these examples , and 4 " long lets say rather then thick ?
Flap :
https://en.wikipedia.org/wiki/Flap_steak
and
https://www.finecooking.com/article/meet-your-new-favorite-steak
note the whole flap lower in the article.
if your roast is more like the first ref , just 4 " long Id go for 130 F 8 - 12.
I also didn't know that its in New England that the flap meat mentioned is called sirloin tips
find what this flap is called in your areas , and get it whole , and on sale if you can
as its Good to Great Eating
flap , whole , looks like this :
cut across the grain , as its usually packed , looks like this
