function calcuulate(){
	var gears = new Array(9);
	var bestRatios = new Array(10);
	var currentSpot = 0, tempRatio = 0.123, difference = 0.123,answerString = "", ans2string = "";
	var tempThresh = document.getElementById('threshField').value;
	if(isNaN(tempThresh) || tempThresh < 0){
		threshold = 0;
		document.getElementById('threshField').value = 0;
	}else{ threshold = tempThresh }
	
	for(i =0; i < 11; i++) {
		bestRatios[i] = 9;
	}

==================

function inc_or_dec(which) {
	var value = new Number(document.getElementById('threshField').value);
	if(isNaN(value) || value < 0){
		value = 0;
	}else if(value >= .05 && which == false){
		value -= .05;
		value = Math.round(value*100)/100;
	}else if(which == true){
		value += .05;
		value = Math.round(value*100)/100;
	}else{ value = 0 }
	document.getElementById('threshField').value = value;
	delete value;
}

======================

		<input type="button" value="&#x2190;" onclick="inc_or_dec(false)">
		<input type="text" size ="5" id="threshField" value="threshold">
		<input type="button" value="&#x2192;" onclick="inc_or_dec(true)">
		<input type="button" id="calcButton" value="Calculate" onclick="calcuulate()" disabled>