central bank official rate is 1thb = 62 mmk. So, I suggest you have to ask your mm bank to make a certificate/letter that prove your account balance is 50 mil kyats and is equivalent to 800k thb . You need to ask them to include both currencies for your balance in that certificate. Anyway, if you can spend extra budget, I recommend you to apply in Ho Chi Minh, Vietnam than Laos. HMC is more flexible in documents checking process.