From the shared Herman Gulch/Watrous Gulch Trailhead, (Exit 218 on I-70 and 3 miles west of Bakerville exit) proceed up the Herman Gulch trail, making sure you don't head off on the trail that leads east over to Watrous Gulch. Within the first mile of steeper switchbacking, the trail begins to emerge out of the forest and into more open meadows where it begins to level off some and you begin to escape the traffic noise of I-70. The trail stays on the north side of the creek all the way to the lake. Soon, you will enjoy good views of your destination - Pettingell Peak. About 2/3 mile from Herman Lake, the trail begins to climb more steeply again through the last trees and leads to Herman Lake. This trail is a very popular Front Range day hike so expect to see large numbers of people on the trail and at the lake.
Note: The trail to Herman Lake can also serve as an alternate approach to The Citadel (UN13,294), Hagar Mountain and Hassell Peak (UN13,215).
From the east side of Herman Lake, work your way north and then west to a bench overlooking the lake. From here, you can pick any line you wish, but here's how we did it: From the bench area, continue west up a shallow gully a few hundred feet to a more level area. Continue traversing the SE face of Pettingell in a westerly direction heading for a saddle SW of the true summit over a mix of intermittent tundra and rubble. Things will steepen again towards the summit. Once you hit the SW ridge, stroll on to the actual rocky summit. In earlier summer, the entire SE face of Pettingell may remain covered in snow. You can enjoy a very nice, not too precarious glissade back down from the summit much of the way to Herman Lake.
For those seeking more of a challenge, the east ridge reported offers some extended and exposed Class 3 challenges. You may want to research that possibility but we have no first hand experience with that route.
Links to other information, routes & trip reports for this peak that may be helpful.