Standards
I have been on both sides of the fence when it comes to creating HMI and PLC standards for big and small organisations.
It honestly goes much further than graphics. To properly asses what you need from your control system graphics you need to understand your business processes in and out.
SCADA these days have many stakeholders depending on how your organisation operated. For one site we have currently i can count 26 different user groups all with different requirements and complexity.
There is no handbook that is going to give you these answers, this is something you need to asses and understand. Its up to you to determine what your stakeholders require, an integrator develops code , he doesnt understand each business he does work for.
From a guidelines perspective i think ASM and the High Performance HMI handbook make the most sense. What people tend to forget is the alarm management, plant specific processes and procedures. I would spend as much time looking at the correct analysis tools for this to assist in keeping the SCADA usable and intuitive for a controller.
Other things to consider is if its a green fields/brown field project. Do you have the freedom to do something brand spanking new?
In the base of brown fields i think leveraging things that are known to your personnel are the best way to go. Even if its not perfect its what the business knows. And believe it or not in our industry you struggle to teach an old dog new tricks.
Introducing something completely new will be a hard sell , i have seen this over and over. Half the battle is convincing the stakeholders.
Operator Training
What learning curve will be involved in getting the best out of the standards you create for your industrial control system.
Engineering Training
How well will you maintenance personnel adapt to what you are creating?
Mass development
How easy is it to roll out new code/screens/blocks, does your integrator provide tools to build code structures?
Performance
Make sure your integrator scales his work up. You wont see a delay with one graphic but put a 1000 on a page and see the difference. This just helps them iron out "heavy" code objects/graphics. Performance always come at the end of a project never at the start.
My last comment is probably the most important. While you are creating these Graphics/Standards blocks whatever. Always and i mean always engage your clients (stakeholders). If they feel they had input into the design it will be easier to accept and adapt to the graphics.