To make the point better:
Week 1. Work in the factory Monday to Thursday. Work in the Bangkok office Friday. Return to work in the factory for 8am Monday morning.
Week 2. Work in the factory Monday to Saturday. This is the weekend when you can cover for any sales trips if required. Its up to you if you want to travel home on Saturday to be in Bangkok on Sunday. However if there is a sales client visit on the Sunday this would have to be covered and you would need to organise with Boontham and Jom who is going to do it - I would expect you all to be fair and take turns equally. Return to work in the factory 8am Monday morning.
Week 3. Work in the factory Monday to Thursday. Work in the Bangkok office Friday. Return to work in the factory for 8am Monday morning.
Week 4. Work in the factory Monday to Saturday. Again, this is the weekend when you can cover for any sales trips if required. Its up to you if you want to travel home on Saturday to be in Bangkok on Sunday. However if there is a sales client visit on the Sunday this would have to be covered and you would need to organise with Boontham and Jom who is going to do it - I would expect you all to be fair and take turns equally. Return to work in the factory 8am Monday morning.