The nature of in play means odds can and do change very quickly. Be grateful you get the warning as at least one book I can think of would take the bet at the adjusted odds.
It is bet to get a feel for correct lay amounts and do it automatically in your head. I would lay £105 per £100 stake immediately then work out my profit once I'd covered the stake
"All you need to ask yourself is do you feel lucky? Well do you......Punk!"