Is there anything to fix?
Should I have done anything different?

Actually there is,
1st of all, thank you for making this mod as GUI bar thing on top than as shortcut button.
My shortcut already full; there is another mod that add calculator functionality, but it come as shortcut button
(plus i don't need the other functionality that mod offer)

But, just in case, you should add option to make this mod appear as GUI thing, or as shortcut, or both in the mod settings; for those that want it.

2nd feature, (but i think this is a mod api limitation since the other calculator can't do this as well)
Can we use keypad directly when inputting in calculator ?
Assuming the calculator is "activated", so it doesn't interfere with other mod that use the same keypad bind.

3rd feature.
The output box/display, can you make an option the font is bigger with contrast color.
I have hard time reading the font / number; not your mod fault, it's just me having slight eyesight problem.
To be clear, i can read the input button just fine, but have to take a closer look with the output display.

You're welcome.

1) Good idea.
2) I think you're right. AFAIK Factorio doesn't support GUI events like that. A few keyboard shortcut might work however. I'll investigate further.
3) Good idea.

0.0.4 released.

Thanks , i just check it, it's tied to map setting ?
Not per player ?
Does that mean i need to start a new map before a new setting take effect ?

Also, Max is 18 ?
Request color can be changed ? To something like this ?

https://t4.ftcdn.net/jpg/01/82/98/13/240_F_182981370_2RCvhUofNgmyWumQ0akXMPIjRziLJLcB.jpg
This is what i mean couple days ago by having contrast. (really helpful due bad eyesight)

Change it when you like. Its in the map setting so you can set it before or in game.
Max is 18 because of an issue when its larger. The horizontal alignment becomes strange under certain circumstances. Basically you don't see the -ve sign when one should be visible. The mod itself supports up to 30 but I've clipped it to 18 for now.
Color and font size increase is not complete yet. Next release.