OK, thats interesting H. The controller looks to be right, saves doing all that from scratch, and its ASCOM compliant, bonus.
Linac linear actuators are popular in OZ
http://www.linak.com.au/techline/applications.aspx
I had a poke at a clamb shell
https://www.youtube.com/watch?v=xs8boe4xruM and used rotary control as I couldnt work out how to use linear actuators. If an actuator opens to vertical, how do you move passed that?. Im a bit thick with mechanicals, im sure the answer is obvious and others will chime in.
One warning. What killed my design stone dead in actual site use, was it was a wind sail. A bit of wind, and things got wobbly and out of control quickly, just when it needed to close with a storm coming. If wind is possible near the shed, open/close it manually on a windy day and check how it behaves/how much energy it takes.