Thank you Vinny ,Casa, and Kaytee88 for answering my questions.
My situation has changed to a more complicated turn. My now husband (yes we just got married last saturday! yay!

) has just got a job offer for a much better job and decided to change job. He will continue working for his current company until the end of September because he has to give them one month notice. His new job will start immediately after so no gap in employment. His new job offer 30k a year so it satisfies the finance requirement . my tier 5 visa will run out on 14th of september (before he move to his new workplace). Therefore, i am thinking of still applying to switch under finance calculation category A with his current employment( assuming what happens at the time i hand in the application is what they count for?). I will write a cover letter explaining the situation, his current employment letter, contract and 6 months pay slips, along with his new employment letter and contract. I also state as soon as we get his first month pay slip we will send them it. I wonder if it is enough to cover this situation?
or should i opt to category B 12 months calculation instead?
I have been working with my current employment for 12 months with the first 8 months of 20k/year and the past 4 months of 23k/year
He has been working with his current employment for 6 months with salary of 22.5k/year. With his previous employment he made 15k/year
Is that mean if we go for category B, only my income satisfies the requirement? Therefore it is better to stick with category A calculation instead?
@Kaytee88: Thank you for your advice. I am under tier 5 government authorised exchange so i am a little bit different from you. We actually got married in the UK and have been warned about the 70 days rule . A week after handing in our marriage notice, we each got a letter from the home office states that they wont investigate our case. However, they says they may carry investigation when we apply for spouse visa later. I hope this may help other people.
Many thanks